In a conventional optical printer for producing an image on a photosensitive object by using the light which has passed through an optical shutter from a light source, the amount of the light of the light source is corrected by using the direct light from the light source. However, the amount of the light from the light source varies after passing through the optical shutter. The amount of the light which has passed through the optical shutter does not always vary linearly according to the variation of the amount of the light of the light source. For this reason, the amount of the light which is irradiated directly to a photosensitive object and contributes to the produced image cannot actually be controlled well by controlling the amount of the light of the light source.
On the other hand, when an image is produced on a photosensitive object, the exposure starting position must be decided accurately. Hence, in the conventional optical printer, a sensor for detecting the position of the photosensitive object is provided near the position to start the conveying of the photosensitive object, and the start of the conveying of the photosensitive object is detected based on the output signal from the sensor, and then the time that will elapse before the photosensitive object will reach the exposure starting position, and so on, are presumed to decide the timing of the starting of the exposure. However, the conventional optical printer has a defect that the photosensitive object is not conveyed accurately, the time when the photosensitive object reaches the exposure starting position varies, and thereby the exposure starting position on the photosensitive object is different. As a special sensor must be provided to detect the start of the conveying of the photosensitive object, the cost increases accordingly.
Furthermore, the surface of the instant film in which self-developer is stored is uneven because of the storing portion in which the self-developer is stored. In an optical printer for exposing and developing such an instant film, when the self-developer is squeezed out of the storing portion by the developing device, if the conveying speed at which the instant film is conveyed is increased, the conveying speed fluctuates due to the unevenness of the surface of the instant film. For this reason, the instant film is conveyed with a constant low speed. As a result, there is a problem that much time is required from the exposure and development to the output of the instant film.
In accordance with an aspect of the present invention, an optical printer forming images by exposing a photosensitive object comprises: a light source irradiating a light; an optical shutter producing exposure light for exposing the photosensitive object by controlling a transmission of the light from the light source; and a photo detector outputting a signal by detecting the exposure light which exposes the photosensitive object.